We intend to study the effects of delta-9-tetrahydrocannabinol (THC, marihuana) on aircraft pilot performance. To date, only one study has examined the effects of THC intoxication on piloting. This study used ten subjects and observer-based performance ratings. No measurements were made of blood or urine THC metabolites in relation to performance, nor were performance decrements related to changes in more basic psychological components. Given the widespread recreational use of THC in both private and military sectors, obtaining more detailed data about its effects on pilot performance is warranted. We propose to examine the effects of THC on pilot performance using a computerized Flight Simulator specifically designed and calibrated for laboratory research. The independent and dependent variables studied in these experiments were selected on the basis of existing findings and their ecological validity, i.e., their relevance to actual pilot performance. The dependent measures are collected on a set of cognitive, behavioral, and flight performance variables. All four experiments share a subset of three independent variables: a) THC dose level, b) task difficulty, and c) delay period between THC dose and test (up to 24 hours later to test for "hangover" effects). Experiment I addresses the effects these three variables plus the pilots' level of recreational use of THC have on performance. Experiment II asks what effects these three variables plus pilot fatigue have on performance. Experiment III looks at the effects of these three variables plus flying experience on pilot performance. Experiment IV asks what effects these three variables plus age of pilot have on performance. In these experiments we will be particularly concerned with the possible interactions of the four variables. Presently, little is known about the effects of THC on piloting, even though research, especially on driving performance, suggests that the drug may have its greatest and longest lasting effects on the performance of cognitively and behaviorally complex tasks. Just as automobile driving performance research revealed effects of THC not found in studies of simpler tasks, we expect studies of the even more complex task of piloting to reveal effects not found in driving studies.